Portland cement is a very important building material. It was first discovered in England. It got its name because on setting, it hardened to a stone-like mass and was compared to the famous Portland Rock of England. It is a mixture of calcium and aluminium silicates with gypsum.
| The approximate composition of cement is |
|---|
| Calcium oxide (CaO) = 50 - 60% |
| Silica (SiO2) = 20 - 25% |
| Alumina (Al2O3) =5 - 10% |
| Magnesium oxide (MgO) = 2 - 3% |
| Ferric oxide (Fe2O3) = 1 - 2% |
| Sulphur trioxide (SO2) = 1 - 2% |
Raw Materials
Limestone and clay are the chief raw materials used in the manufacture of Portland cement.
- Limestone provides calcium oxide.
- Clay gives silica, alumina and ferric oxide.
Note: If lime is in excess, the cement cracks during setting and if it is less, the cement is weak in strength. Free lime is present in negligible quantity in cement, Almost the entire quantity of lime is present in the combined state as calcium silicates and aluminates.
Manufacture
Limestone and clay are first crushed separately. They are then mixed together in the required quantities and ground together finely. This grinding can be done either by the dry or the wet process. Wet process is suitable if limestone and clay are soft. If the raw materials are hard, dry process is preferred. Dry process is cost effective because fuel consumption is less when the raw materials are burnt in a rotary kiln.
Process
- The clay is first washed with water in wash mill to remove the foreign materials.
- Powdered lime is added to clay and the mixture is finely ground and homogenised.
- The clay-lime slurry (40% water present) or the dry pulverized mixture (in the case of dry process) is introduced into the upper end of the rotary kiln.
- The rotary kiln is a brick-lined, inclined, steel rotating cylinder, 150 - 200 feet long and 10 feet in diameter.
- Water evaporates at the upper end of the kiln with the help of hot gases, in the wet process.
- The dry material gravitates down and comes into contact with a flame produced by burning powdered coal.
Upper Part of the Kiln
Middle Part of the Kiln
Temperature:1000oC. Limestone decomposes to calcium oxide.
Lower End of the Kiln
Temperature:1600oC. The partly fused and sintered mixture undergoes a series of chemical reactions to form calcium aluminates and silicates.
All these reactions are exothermic releasing heat. Proper temperature control is a must to avoid excessive vitrification (conversion into a glasslike substance by heat) of the mass.
The resulting mixture of all these silicates and aluminates is called cement clinker. After cooling, it is mixed with 2-3% gypsum and ground to a fine powder. Gypsum slows down the setting of cement so that cement is adequately hardened.Setting Properties of Silicates and Aluminates
- Tricalcium silicate - sets quickly and develops considerable strength in a few days. Constitutes 50% of cement.
- Dicalcium silicate - sets slowly and becomes strong after a month or so.
- Tricalcium aluminate - sets instantaneously in the presence of water. Cement's internal strength is mainly because of this.
- Tetracalcium aluminoferrite sets rapidly but not so fast as tricalcium aluminate.
Setting of Cement
When mixed with water, cement sets to a hard mass. It first forms a plastic mass which hardens after some time due to 3-dimensional cross-links between the --Si-O-Si-- and --Si-O-Al-- chains. The first setting occurs within 24 hours whereas the subsequent hardening requires a fortnight, when it is covered by a layer of water. This transition from plastic to solid state is called setting.Reactions Involved in Setting of Cement
- On hydration silicates and aluminates of calcium get converted to their respective hydrated colloidal gels.
- At the same time, hydrolysis precipitates calcium hydroxide and aluminium hydroxide.
This calcium hydroxide binds calcium silicate particles together. On the other hand, aluminium hydroxide fills the interstices (an intervening space) rendering the mass impervious (not affording passage to a fluid).
- Role of gypsum - Gypsum reacts with tricalcium aluminate.
Note: The fast-setting tricalcium aluminate is removed to slow down the setting process. A quick setting will give rise to crystalline hydrated calcium aluminate. A slower setting yields the colloidal gel that imparts greater strength to the set mass. Thus gypsum helps in regulating the setting time of cement.
Concrete
It is a mixture of cement with sand and ballast (a mixture of coarse and fine aggregate for making concrete). Water is added to get a proper consistency. When set, a hard solid mass is formed.Uses
- Concrete is used in foundations, flooring and walls.
- It is also used in paving roads.
- It also finds use in building roofs, piers for bridges, dams and canals.
